USB (Universal Serial Bus) Type-C cables, plugs, and receptacles form a standardized electrical interconnection for transmitting electrical power and data. In part, the USB Type-C (USB-C) standard covers a 24 pin fully reversible plug and mating receptacle, referred herein as a USB-C connector. The USB-C standard also covers captive and plug-in cables. USB-C cables are used to connect power sources, such as a power supply in a cabin for an aircraft, to Personal Electronic Devices (PEDs) such as computers, tablets and smart phones.
The standardized Universal Serial Bus Power Delivery specification (USB PD) includes an ability to negotiate power contracts between a power source and a power sink such as a PED. Power contracts are established when the power source advertises a set of source capabilities, the power sink choses one capability, and then the power source confirms the power sink's choice. The capabilities are defined by data structures called Power Data Objects or Augmented Power Data Objects, referred to herein as PDOs. Under USB PD specifications, PDOs may specify a variety of power source characteristics. Such characteristics include a fixed output, a battery-powered output and programmable outputs. The outputs typically have voltages up to 20V, currents as high as 5 A, and resulting power levels up to 100 W. By advertising different PDOs, the power source can control the maximum power drawn by a power sink.
The following definitions are commonly employed with USB PD specifications and are applied herein:
Power Supply—An electronic device that converts one form of electrical energy to another. As disclosed herein, the power supply converts electrical energy to a USB-C PD specified voltage and current level.
Port Controller—A device that handles the communication in and out of a source port or sink port and the computing resources allocated to managing the port.
Source Port—A functional block of circuits that comply with the USB-C and USB PD specifications to provide power. The source port includes a power supply and a port controller
Sink Port—A functional block of circuits that comply with the USB-C and USB PD specifications that use power provided by a source port.
All USB source ports dissipate heat, and therefore have internal temperatures and surface temperatures that exceed the ambient temperature. For safety reasons, a typical USB source port will disable the output if an internal temperature reaches a pre-determined limit. While this provides safety protection, it can be an inconvenience to a user of the source port, such as the owner of a PED. Disclosed herein are a system and a method to negotiate the power level of a USB PD capable source port, to keep the internal temperature within safe limits without having to disable the output.
One example of negotiated power levels is found in United States Patent Application Publication No. 2010/0285828 A1, titled “Throttling Transmit Power in a WWAN Device Based on Thermal Input,” by Panian et al. This publication discloses a wireless wide area network (WWAN) communication system having access terminals located remotely from a base station. A temperature sensor determines an expected increase in temperature in an access terminal based on a target transmit power. A negotiating module then communicates with the base station to set the target transmit power sufficiently low to avoid overheating components of the access terminal.